The present invention relates to the generation of a signal that is proportional to the noise included in an audio signal transmitted as frequency modulated stereo-multiplex signal.
In fm-broadcasting a stereo-multiplex signal is frequency modulated. The stereo-multiplex signal consists of a stereo-sum signal and a stereo-difference signal. The stereo-difference signal is amplitude modulated with suppressed carrier. To allow a coherent amplitude demodulation of the stereo-difference signal at the receiver, a pilot carrier with half the AM-carrier frequency is added to the stereo-multiplex signal.
The stereo-sum signal and the stereo-difference signal are defined by
ms(t)=al(t)+ar(t)
md(t)=al(t)xe2x88x92ar(t)
The stereo-multiplex signal is defined by
mstmux(t)=ms(t)+sin (2 xcfx89pilt) xc2x7md(t)+Apilxc2x7sin(xcfx89pilt)
The stereo-multiplex signal is frequency modulated:             S      FM        ⁡          (      t      )        =            A      FM        ⁢          cos      ⁡              (                                            ω              c                        ⁡                          (              t              )                                +                      Δω            ⁢                                          ∫                                  -                  ∞                                t                            ⁢                                                                    m                    stmux                                    ⁡                                      (                    τ                    )                                                  ⁢                                  xe2x80x83                                ⁢                                  ⅆ                  τ                                                                    )                  with    ⁢          xe2x80x83        ⁢                                                      xe2x80x83                        ⁢                                          ω                c                            ⁢                              :                                                                          carrier            ⁢                                          xe2x80x83                            ⁢                              xe2x80x83                                      ⁢            frequency                                                            Δ            ⁢                          ω              _                        ⁢                          :                                                            frequency            ⁢                          xe2x80x83                        ⁢            deviation                                ⁢      
  
At the receiver side the frequency modulated stereo-multiplex signal is frequency demodulated and stereo-demultiplexed to calculate the left and right audio signal.
For the stereo demultiplexing, the stereo demultiplexer needs to recover the 2nd harmonic of the pilot carrier. Therefore, a PLL locks to the pilot carrier and generates the 2nd harmonic of the pilot carrier. The 2nd harmonic, that is locked in phase to the pilot carrier is needed for the coherent amplitude demodulation of the stereo-difference signal.
FIG. 6 shows the basic functionality of a state of the art stereo-demultiplexer. For the sake of simplicity the noise nb(t) added to the frequency modulated stereo-multiplex signal SFM(t) on the transmitter side, the receiver side and within the transmission channel is shown to be added to the frequency modulated stereo-multiplex signal SFM(t) by way of an adder 10 just before the frequency demodulator 11 of the stereo-demultiplexer shown in FIG. 6. Therefore, the frequency demodulator 11 outputs a stereo-multiplex signal u(t) that consists of the stereo-multiplex signal mstmux(t) as generated on the transmitter side and additionally an added noise component v(t) that is the frequency demodulated noise signal nb(t). On basis of this stereo-multiplex signal u(t) a PLL-circuit 2 generates the 2nd harmonic of the pilot carrier, i. e. a signal 2xc2x7sin(2xcfx89pilt), which is needed for the coherent amplitude demodulation of the stereo-multiplex signal u(t) to gain the stereo-difference signal ud(t). This coherent amplitude demodulation is performed by way of a demodulator 12 which receives the stereo-multiplex signal u(t) at its first input and the 2nd harmonic of the pilot carrier at its second input. The output signal of the demodulator 12 is input to a filter 9 which outputs the stereo-difference signal ud(t) that consists of the stereo-difference signal md(t) generated at the transmitter side plus an additional noise component vd(t). A stereo-sum signal us(t) comprising the stereo-sum signal ms(t) plus an additional noise component vs(t) is generated by a lowpass filtering of the stereo-multiplex signal u(t) with a lowpass filter 8 that receives the output signal of the frequency demodulator 11. The left audio signal is calculated by an addition of he stereo-sum signal us(t) and the stereo-difference signal ud(t). The right audio signal r(t) is calculated by a subtraction of the stereo-difference signal ud(t) from the stereo-sum signal us(t). The left output channel consists of of the left audio signal l(t) and a noise component vd(t)+vs(t) and the right audio channel consists of the right audio signal r(t) and a noise component vs(t)xe2x88x92vd(t).
Therefore, without consideration of the noise nb(t) introduced in the transmission chain, the stereo-sum signal ms(t) is generated by a lowpass filtering of the stereo-multiplex signal and the stereo-difference signal is generated by a coherent amplitude demodulation of the amplitude modulated stereo-difference signal. The left and right audio signals l(t) and r(t) are calculated by addition and subtraction of the stereo-sum signal and the stereo-difference signal:
r(t)=ms(t)xe2x88x92md(t)=(al(t) +ar(t))xe2x88x92(al(t)xe2x88x92ar(t))=2ar(t)
l(t)=ms(t)+md(t)=(al(t) +ar(t))+(al(t)xe2x88x92ar(t))=2al(t)
The detection of noise in a frequency demodulated signal is very important in the receiver. Depending on the noise level, the receiver switches from stereo-detection to mono-detection. In current car receivers the noise detection is done by analyzing the reception fieldstrength. In an optimal reception situation (only Gaussian noise, one path channel), the noise detection by analyzing the fieldstrength is reliable. If the fm-signal is received by an active antenna, the noise detection by analyzing the reception fieldstrength is not possible without knowing the characteristics of the active antenna. In case of multipath reception the detection of noise in the audio signal by analyzing the fieldstrength is very unreliable. Therefore, most of todays portable receivers include a multipath detection circuit, which is based on the detection of an amplitude modulation of the frequency modulated RF-signal. This kind of multipath detection is unreliable, especially the calculation of noise in the frequency demodulated audio signal is very unreliable. The third kind of noise in an audio signal, noise peaks, generated for example by sparks are detected in most of todays portable receivers by a highpass filtering of the frequency demodulated signal. This kind of noise peak detection is also unreliable.
Therefore, it is object of the present invention to calculate a reliable information of the noise power in the audio signal.
This object is solved by a method to generate a signal that is proportional to the noise included in an audio signal transmitted as frequency modulated stereo-multiplex signal according to independent claim 1, a stereo-demultiplexer according to independent claim 4 and a noise indication circuit that generates a signal that is proportional to the noise included in an audio signal transmitted as frequency modulated stereo-multiplex signal according to independent claim 6. Preferred embodiments thereof are respectively defined in the dependent sub-claims.
According to the present invention the characteristics of the stereo-multiplex signal itself is used for the calculation of the noise power in the stereo-difference signal. The so calculated noise power is a very reliable information that can be used for the sliding stereo-mono transition, noise blanking and a de-noising of the stereo-difference signal as well as for the general indication of the signal quality of the broadcasted stereo-multiplex signal for other purposes.